An image forming apparatus includes an latent image carrier configured to carry a latent image thereon, a developing unit disposed facing the latent image carrier to develop the latent image with developer, a developer container containing the developer and attachable to the image forming apparatus, a developer detector configured to detect the presence of the developer in the developing unit, and a developer supply controller. The developer supply controller prohibits supply of the developer from the developer container to the developing unit when the developer detector detects that the developer is present in the developing unit.